This invention deals generally with plastic injection molds and more specifically with a gate insert within the mold with the gate insert constructed to reduce thermal stress in optical discs produced by such molds.
The production of optical discs, such as compact discs, DVD discs and CDR discs involves molding a thermoplastic or thermoset into the disc that the consumer ultimately purchases. These optical discs are cast in individual molds, and must meet stringent criteria for optical quality. The final optical quality is greatly determined by the internal stress to which the disc is subjected during its manufacture. More specifically, the internal stress is created by temperature variations within the casting mold.
The molding process involves injecting hot thermoplastic or thermoset liquid at a location at the center of a disc-shaped mold and permitting the material to flow radially outward into the other more remote portions of the mold. Optical disc molds typically include a cylindrical annular hub bushing called a gate insert located near the center of the mold, into which a sprue bushing is inserted. The hot liquid polycarbonate is furnished through the sprue bushing, and the annular gate insert, the portion of the mold which is subjected to the hottest temperatures, has circumferential channels formed into its surface through which cooling water flows in order to reduce the temperature of the gate insert. The outer flat portions of the mold, the surfaces which form the majority of the upper and lower disc surfaces, are also typically water cooled.
The typical annular gate insert is constructed of steel because the gate insert is subjected to significant wear by erosion from the liquid polycarbonate and contact with other moving parts of the mold assembly. When the wear on the gate insert is great enough, the entire manufacturing process must be shut down to replace the gate insert. Unfortunately, however, the steel used to reduce wear of the gate insert is a relatively poor heat conductor, so that even with the gate insert exterior cylindrical surface cooled by water, the end surface which is the mold boundary surface and is in contact with the hot thermoplastic or thermoset retains heat and causes optical distortion in the finished disc.
When the liquid polycarbonate is injected into the center of the mold it is very hot, and transfers some of that heat to the gate insert mold boundary surface. Due to insufficient cooling with the conventional steel inserts, a temperature difference results between the inner radius of the disc at the gate insert and the outer radius of the disc. This temperature difference causes residual physical stress in the finished disc which is due to non-uniform cooling and retained heat in the gate insert.
It would be very beneficial to have gate inserts available which have very good thermal conductivity but are also able to resist damage from operation in the molds.
The present invention constructs gate inserts in the same configurations in which they were previously made, but uses a different combination of materials. The gate insert of the present invention is essentially constructed of copper or aluminum or a copper or aluminum based alloy such as copper-beryllium, copper-aluminum, copper zinc, or aluminum-zinc-magnesium. However, such materials by themselves, although excellent heat conductors, can not be used as gate inserts because they are far too soft and would wear extremely fast.
Therefore, in the present invention the exterior surfaces of the base structure of the gate insert are coated with a thin film physical vapor deposition (PVD) or chemical vapor deposition (CVD) coating which furnishes an extremely hard coating to protect the surfaces from wear. Furthermore, since the PVD or CVD coating is only 0.25 to 10 micrometers thick, it has virtually no effect on the heat conductivity between the water cooled channels and the mold boundary surface in contact with the optical disc. Typical materials used for the physical vapor deposition and chemical vapor deposition coating are titanium nitride, titanium carbide, titanium carbonitride, titanium aluminum nitride, chromium nitride, chromium carbide, zirconium nitride, zirconium carbide, and metal diamond-like-carbon or multiple layers of such materials. The application of such coatings to metal parts is well understood by those skilled in the art.
The invention thereby furnishes gate inserts with the heat conductivity of the best heat conductive metals available and surface hardness and wear resistance which not only exceeds that of the metal used, but actually surpasses the wear resistance of the steels conventionally in use for gate inserts for optical disc molds.
The results of the invention are best demonstrated by the measurement on finished optical discs of the property known as birefringence. Birefringence is a signal measurement of the optical clarity of an optical disc. It is also an indicator of the residual physical stress in the disc. The test is made by taking optical measurements starting at the inner radius of the disc, which is determined by the location of the gate insert, and progressing outward along greater radial distances to the outer edge of the disc.
Although the ideal goal for birefringence is a reading of zero, industry specifications state that for audio compact discs and DVD disks birefringence should be less that +/xe2x88x9250 nanometers, while the standard for CDR discs is less than +/xe2x88x9230 nanometers. Conventional steel gate inserts often produce discs which are out of specification.
Using the gate insert of the present invention, discs are consistently produced with birefringence readings within the range of +/xe2x88x9215 nanometers.